hot subdwarf

A hot subdwarf is a helium burning star, with a helium core
mass of about 0.50 Msun, that is covered with a very thin hydrogen
shell of about 0.02 Msun. Hot subdwarfs are immediate progenitors
of white dwarfs. There are several types,
recognized by their optical spectra, as listed in the table. Types sdB and
sdOB are thought to be extremely blue horizontal
branch stars that evolved (like HBB stars) from red
giants. They form a relatively narrow sequence at the blue end of the
horizontal branch (HB), and are therefore often referred to as extended
horizontal branch stars (EHB). They differ from normal HB stars in that
they don't evolve to the asymptotic giant branch.
By contrast, sdO stars are in a post asymptotic giant branch phase of evolution.